1. Field of the Invention
The present invention relates generally to optical metrology and, more particularly, to non-contact surface contour measurement systems and methods.
2. Related Art
Contoured, free-form surfaces and complex part geometries present special manufacturing challenges that have made conventional dimensional metrology approaches unsuitable for providing the necessary degree of control over the manufacturing process to insure that the workpiece dimensions are consistently within tolerances. Contoured, free-form surfaces, which generally have shapes that curve in the three coordinate axes, may be found on automobile bodies and jet engine blades; devices shaped for human use such as telephone hand sets, computer keyboards and tennis rackets; and artificial joints, such as replacement hips and knees. Complex geometries may be found on workpiece surfaces having very small dimensions, such as printed circuit boards, electronic assemblies and intricate machined parts.
In modern metrology it is important to determine not only the true position of a workpiece feature, but also size, run out, flatness, and other dimensional form characteristics of the entire workpiece that can indicate, for example, how well the part will perform as a component in a larger assembly, and how well intricate parts of a larger component have been assembled. Unlike prismatic shapes which require a minimum number of data points to establish dimensional information such as size and location, accurate form measurements require the compilation of a massive number of data points. For example, the diameter of a circle can be defined with a minimum of three data points. Form measurement of a circle, on the other hand, could require as many as 4,000 data points depending upon the level of definition desired.
Coordinate measuring machines (CMMs) have traditionally been used to gather dimensional data for inspection and process control purposes. In particular, scanning CMMs have been developed in response to the need to measure contoured, free-form surfaces. These CMMs automatically collect a large number of data points to define the three-dimensional shape and form of a workpiece or a workpiece feature, and use the information in conjunction with, for example, a CAD system to provide insight into the manufacturing processes. Two common scanning methods are continuous and stitch scanning. Continuous analog scanning is generally performed using an analog scanning probe or a passive probe. Analog probes are used on direct computer control (DCC) CMMs to measure, for example, changes in a workpiece""s surface elevation as a proportion of the measuring tip""s deflection over its full range. Continuous analog scanning typically sends an analog signal to a computer which processes the signal, converting it into a stream of discrete data points. However, continuous analog scanning probes experience large deflections with minimally-applied forces. Even when used with DCC CMMs, the displacement of analog scanning probes cannot be sufficiently controlled to yield the necessary accuracy to measure intricate and complex workpiece features.
Passive probes, which are generally used in manual CMMs, typically have solid shafts and a large probe tip diameter to reduce errors due to static bending. Unfortunately, conventional passive probes (also referred to as solid or hard probes) have a diameter that is generally too large to be used to measure small features of a workpiece. Furthermore, the diameter of passive probe shafts cannot simply be reduced to accommodate these smaller dimensions. Passive probes have deflections that significantly increase with a corresponding decrease in the diameter of the probe shaft. Thus, reducing the diameter of the passive probe shafts results in significant probe tip deflections. Another drawback of conventional passive probes is that the accuracy of the probes is dependent upon the operator and the speed at which the measurements are performed. As a result, passive probes in manual CMMs are incapable of meeting the requisite accuracy to define form errors of small workpiece features. Also, passive probes cannot be used on DCC CMMs because in such applications the force experienced by the probe is restricted.
In stitch scanning, data is acquired using a switching or touch trigger probe which often includes an electronic or electro-mechanical trigger that generates a signal each time the probe touches a surface of the workpiece. When the probe contacts individual points on a workpiece surface, the probe tip deflects and a switched signal is generated that causes the CMM to provide discrete data points to a controller. The probe tip must then be lifted to break contact with the workpiece surface, moved slightly, and contact reinitiated at another location along a measurement line to collect another measurement point. This approach may be effective for applications requiring a minimum number of data points to be gathered, such as where fairly smooth two-dimensional surfaces prevail. However, for three-dimensional form measurements, the single-point measurement technique, which generally collects data at a maximum rate of approximately 50-60 points per minute on a DCC machine and 20-30 points per minute on a manual scanning machine, cannot practicably collect the requisite number of data points to accurately define form errors. The time required to obtain such measurement points using a switching probe increases the cost of making such measurements due to the increased labor costs and adverse impact on the manufacturing speed. In addition, the time required to make such measurements increases the potential for measurement errors due to, for example, heat expansion during the measurement process.
One conventional approach that has been developed to overcome these limitations of CMMs includes the formation of an image of the workpiece, typically acquired by a video camera. The image is digitized and stored in a computer memory as a set of pixels. The computer then analyzes the image, such as by comparing it, pixel-by-pixel, with a stored reference image. However, there are drawbacks to such conventional techniques. For example, processing of a stored image requires a very large number of calculations. Even with high speed digital computers, processing of many such stored images requires considerable time, thereby limiting the ability of the system to generate immediate, real-time results. Also, the images are often of poor quality due to the inability of such conventional system to accurately obtain high resolution images of the workpiece which, in turn, reduces the accuracy of the resulting measurements.
More recently, other non-contact measurement techniques have been developed to measure surface contours. Such systems have been used to measure workpieces having complex surface configurations, contoured, free-form surfaces, and other complex workpieces such as printed circuit boards and the like. Typically, a single point range sensor using optical triangulation techniques is used to perform non-contact measurements. An illumination source projects a defined area of light onto the surface to be measured. Reflections received from the surface are used to form an image of the light reflected onto a light-sensing detector. As the distance from the sensor to the workpiece surface changes, the position of the reflected image on the detector plane shifts. This lateral shift of image position on the detector is used to measure the distance between the sensor and the surface, thereby providing the dimension of the measured surface.
A drawback to such conventional optical triangulation techniques is that in order to obtain high accuracy, the detector must be able to resolve small lateral shifts in the image position on the detector. This generally requires a high magnification in the direction of travel of the reflected image. However, the sensor is typically separated from the workpiece by a large stand-off distance. As a result, the sensor has a limited range of measurement and must be adjusted in position relative to the workpiece to retain the workpiece within the measurement range. Furthermore, such conventional systems are typically slow, increasing the cost of performing such measurements.
What is needed, therefore, is an apparatus and methodology that is capable of quickly and accurately obtaining multiple measurement points for determining the dimensional form characteristics of contoured, free-form surfaces, highly complex surfaces and workpiece features having very small dimensions.
The present invention is an improved apparatus and method for rapidly and accurately measuring surface contours of a workpiece. In one aspect of the invention, an optical metrology system for measuring a contour of a workpiece surface is disclosed. The system includes a multi-wavelength light projector that projects a wavelength-varying collimated light beam onto the surface of the workpiece. The collimated light beam has a plurality of substantially parallel light rays, each of which has a predetermined wavelength. The wavelength of the plurality of light rays varies in a predetermined manner across a width of the collimated light beam. A wavelength-discriminating detector determines an intensity of light reflected from the workpiece surface and detects wavelength-specific characteristics of the received reflected light. Significantly, the wavelength-specific characteristics of the reflected light are related to the distance of the workpiece surface from the detector.
Specifically, the multi-wavelength projector includes a collimated light source that generates a collimated light beam. A wavelength filter in the multi-wavelength projector is operatively positioned adjacent to the collimated light source to filter predetermined wavelengths of certain ones of said plurality of light rays to generate the wavelength-varying collimated light beam.
More specifically, the collimated light source includes a light source that emits light having a plurality of wavelengths, and a collimator constructed and arranged to collimate the emitted light so as to generate the collimated light beam. Preferably, the light source is a white light source. In one embodiment, the collimator is a cylindrical lens. In an alternative embodiment, the collimator is a parabolic mirror.
In one preferred embodiment, the wavelength filter dynamically filters the collimated light beam. In such embodiments, the wavelength filter comprises a means for dynamically controlling a plurality of optical filters such that the filters temporally filter the collimated light beam. In one embodiment, the controlling means is a tunable liquid crystal display filter or other device that electronically controls the plurality of optical filters. Alternatively, the controlling means may mechanically control a plurality of physical optical filters. The wavelength filter may filter the collimated light beam such that the wavelength-varying collimated light beam comprises light rays with wavelengths that vary linearly or non-linearly across the light beam. In one particular embodiment, the wavelength filter comprises a color filter that filters the collimated light such that the wavelength-varying collimated light comprises light rays which vary from approximately 400 to 700 nanometers (nm). In one particular embodiment, the detector is constructed and arranged to discriminate to one (1) nm of resolution of light reflected from the workpiece surface.
In one alternative embodiment, the wavelength filter includes two electronically-controlled color filters operatively arranged to successively filter the collimated light beam. In this embodiment, a first filter passes a single, predetermined wavelength of the collimated light. The predetermined wavelength is temporally varied using any well known electronic control. A second electronic filter is operatively positioned to receive the filtered light. The second filter passes a range of predetermined wavelengths through a narrow region of the filter which is electronically controlled so as to be positioned at different locations along the second filter. The position of the narrow region along the second filter is controlled synchronously with the change in the wavelength passed by the first filter. This results in the wavelength filter generating a narrow beam of collimated light having a wavelength that varies across its width.
Generally, the wavelength discriminating detector includes a photodetector matrix having a plurality of photosensitive elements forming an array of wavelength-sensitive light detector elements. A light receiver of the wavelength discriminating detector projects light reflected from one or more predetermined locations on the workpiece surface onto the photodetector matrix such that wavelength-specific characteristics and intensity information of the reflected light is recorded by the photodetector matrix.
In one particular embodiment, the light receiver is a fiber chromatic sensor which includes a plurality of optical waveguides each of which receives and projects a portion of the reflected light onto an associated one of the light detector elements of the photodetector matrix. Preferably, the fiber chromatic sensor includes a parallel light receiver coupled to the input end of each of the optical waveguides. The parallel light receiver receives light rays of the reflected light which are substantially parallel with an axis of the input end of the optical waveguide. In one embodiment, the parallel light receiver is a hemispherical lens formed on the input end of the optical waveguide. Alternatively, the parallel light receiver is a variable refractive index light guide having a predetermined length so as to form a parallel light accepting receiver.
The fiber chromatic sensor preferably includes a projection controller operatively coupled to an output end of each of the plurality of optical waveguides. The projection controller projects light transferred through the optical waveguide onto the associated light detector element of the photodetector matrix. Preferably, the projection controller projects the light such that a substantial portion of the projected light is detected only by the associated wavelength discriminating light detector. In one embodiment, the projection controller is formed by selectively removing a portion of the cladding at the output end of the optical waveguide.
In one embodiment, the wavelength discriminating detector includes a photodetector matrix including a plurality of photosensitive elements arranged in columns and rows. Photosensitive elements of each column are configured to detect light having a same range of wavelengths while photosensitive elements in each of the rows detect light having a different range of wavelengths. One or more rows of the photosensitive elements form a wavelength-sensitive detector. A focussing lens projects light reflected from locations on the workpiece surface onto an associated wavelength-sensitive detector of said photodetector matrix. In one specific embodiment, the photodetector matrix includes three columns of photosensitive elements. A first column of photodetector elements detects a broad range of wavelengths centered approximately about the wavelength associated with the color red. A second column of photodetector elements detects a broad range of wavelengths centered approximately about the wavelength associated with the color green. A third column of photodetector elements detects a broad range of wavelengths centered approximately about the wavelength associated with the color blue.
In another embodiment, the wavelength discriminating detector includes a photodetector matrix including a plurality of photosensitive elements arranged to form a plurality of wavelength-sensitive detectors. A light receiver, including a prism and a column of microlenses, projects light reflected from locations on the workpiece surface onto one of the plurality of wavelength-sensitive detectors associated with each surface location. Wavelength-specific characteristics and intensity information of the reflected light is recorded by the wavelength-sensitive detectors.
In one embodiment, the wavelength discriminating detector includes a plurality of photosensitive elements arranged in a column so as to receive light reflected from a substantially parallel surface of the workpiece surface. The wavelength discriminating detector also includes an electronically-controlled filter interposed between the column of photosensitive elements and the object surface. The filter allows a single wavelength of reflected light to pass through the filter to the photosensitive elements. The wavelength of the light which is passed through the filter is temporally varied in a predetermined manner. A column of microlenses arranged substantially parallel with the column of photosensitive elements is also included in the detector interposed between the filter and the object surface. Each microlens in the column of microlenses focuses the light reflected from the object surface onto an associated photosensitive element in the column of photosensitive elements. In one embodiment, the column of photosensitive elements is a line CCD.
In an alternative embodiment, the wavelength discriminating detector includes an array of photosensitive elements arranged in a column so as to receive light reflected from a substantially parallel surface of the workpiece. An electronically-controlled filter is interposed between the array of photosensitive elements and the object surface. The filter allows a single wavelength of reflected light to pass through the filter to the array of photosensitive elements. This wavelength is varied in a predetermined manner over time. An array of microlenses in the wavelength discriminating detector is arranged substantially parallel with the array of photosensitive elements. The array of microlenses is interposed between the filter and the object surface. Each microlens in the array of microlenses focuses the reflected light from the object surface onto an associated photosensitive element in the array of photosensitive elements.
Advantageously, the present invention does not require physical movement of any device or component during a scan of the workpiece surface. Eliminating such operations enables the present invention to provide measurement data significantly faster than the above and other conventional metrology, scanning, imaging or dimensional measurement systems. Furthermore, such static scanning increases the accuracy of the resulting measurements since the errors associated with the physical positioning and control of the measurement system components is eliminated.
Another advantage of the present invention is that, depending upon the complexity of the workpiece surface and other factors, the present invention can obtain measurement data of a workpiece surface as the relative position of the workpiece and the optical sensor assembly continuously changes, thereby decreasing the time necessary to completely measure the surface or dimension.
A further advantage of the present invention is that it generates wavelength-discriminated measurement data, providing for increased measurement accuracy which may be scaled with the invention for different applications. For example, the present invention may be scaled to measure very intricate parts such as components and features of a printed circuit board or electronic assembly. Alternatively, the present invention may be increased in scale to measure larger workpieces such as contoured, free-form automotive body parts.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.